Analytical elements, especially multi-layer analytical elements for quantitative determination of various analytes of biological interest are known from U.S. Pat. Nos. 4,670,381, 4,517,288; 4,258,001; 4,066,403 and 3,992,158. Elements for assaying analytes such as carbon dioxide by enzymatic procedures, aspartate aminotransferase (AST), lactate dehydrogenates (LDH) and alanine aminotransferase (ALT) utilize NADH and measure reflectance density at 340 nm to obtain a quantitative determination of the analyte of interest.
In the case of the analytes just referred to, the measurement is made spectrophotometrically using a reflectometer. In a reflectometer, a beam of energy such as light is reflected off a mirror and subsequently through the support, reagent and registration layers of the element. The light is then reflected back through a filter onto a detector means such as a photodiode. The change in reflectance density (D.sub.R) corresponds to the concentration of the analyte in a test solution. In a number of existing reflectometers the maximum obtainable D.sub.R is about 1.5 in the range 320 to 360 nm. The problem is that in some end point assays, such as the enzymatic approach to measuring CO.sub.2, the maximum D.sub.R generated is about 2.0 at 340 nm. Obviously, using reflectometers with a maximum obtainable D.sub.R of 1.5 results in significant compression of the usable dynamic range for these assays, and therefore degraded precision.